SiC and CSiC Mirrors IABG C/SiC Astrium Boostec SiC

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SiC and CSiC MirrorsIABG C/SiCAstrium Boostec
SiC

• H. Philip Stahl, Ph.D.
• MSFC

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IABG 0.5 m 20 m Rcv Carbon Silicon Carbide

• Study conducted at IABG to determine limits on
  manufacturing lightweight mirror substrates.
• Blank was sand blasted (no grinding or lapping).
• Blank achieved areal density of 7.8 kg/m2.
• Lightweight limit is precision machining on ribs
• Blank has no cladding.
• Blank polished at General Optics
• Figure of ½ wave PV
• Finish of 100 Angstroms RMS
• Mirror tested to 120K at Kodak (Sept 99)
• 4 wave PV (0.45 wave RMS) figure change
• Mirror tested to 30K at MSFC (Apr 01).

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IABG 50-cm Substrate Design
First eigenfrequency 1300 Hz, areal density 8
kg/m² including three-point mount and outer ring


Deformation plotof first eigenmode
Including 1.5 mm for grinding and polishing
R 19998.5 mm is the ROC
Height of circumference edge (4.5 mm after
grinding and polishing)
All dimensions are given in mm.
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IABG Fabrication Plan
Raw material manufacturing (randomly oriented
chopped C/C felt)
Inspection of felt upon arrival from supplier
Programming of the computer controlled milling
machine tool and milling trials with dummy
material
NC milling of raw material mirror substrate
Liquid silicon infiltration and reaction to SiC
in vacuum atmosphere
Surface cleaning by sand blasting
Quality control tests
8/31/98 Page 8
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Substrate after Greenbody Machining
Create carbon fiber block by molding at high
pressure randomly oriented, short, chopped carbon
fiber mixed with a phenolic resin to form a
carbon fiber reinforced plastic blank.
Block is density of compacted styrafoam
(0.65-0.75 g/cm3) and machines like balsa
wood. Carbon fiber block machined in complex
shapes using numerically controlled milling
machines. Complex shapes can be generated by
gluing pieces together with phenolic resin.
Substrate mass 0.582 kg
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IABG Carbon Silicon Carbide

• Raw material can be polished to 10 nm, but
  appears mottled because of both carbon fibers
  and silicon carbide.
• Ion figuring not satisfactory due to bi-phase
  material.
• Blank can be coated with CVD silicon carbide, PVD
  silicon carbide, PVD silicon, or a silicon
  carbide-silicon slurry.
• First three can be polished or ion figured to
  0.1-0.5 nm.
• Slurry can be polished to 3 nm and can be ion
  figured as well.
• Because the fibers are randomly oriented, the
  mirror should have good cryogenic performance

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Substrate after Infiltration
Greenbody put in furnace to carbonize the
phenolic resin. Raise temperature and infuse
liquid silicon into carbon structure and to
create biphase CSiC material. CsiC blank has
very little shrinkage during this process Blank
showed no cracks, density variations or other
inhomogenities. Monolithic structures can be
created by joining segments either in the green
body state or after infiltration to create
monolithic blanks. Join infiltrated segments with
a special glue then put back in furnace to
re-infiltrate with silicon.
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Thermal Vacuum Processing

• Vacuum pressure lt 2 mbar
• Dimensions Diameter appr. 3 meters
  Length appr. 4 meters
• Power supply 700 kWatts
• Max. temperature 2100 C
• Process automation system with inline
  controllable process parameters

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Rear Side View of the Sand Blasted 50-cm C/SiC
Mirror Substrate
Mass 2.477 kg Areal density 8.0
kg/m² Density 2.69 g/cm³ Diameter 500
mm Height 59.0 mm No shrinkage in diameter
observed
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Details of the Rear Side of the Infiltrated 50-cm
C/SiC Mirror Substrate
- Triangular segments - Major ribs -
Cathedral ribs - Mounting provision
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Lessons Learned - IABG

• Machining small rib thickness is a problem.
• Machine Tool Vibration increase web thickness
• Damage can be repaired
• Cathedral ribs are difficult and not necessary.
• Micro-sand blast cleaning works.
• Handling of large green-body parts requires
  special tooling.
• Make webs thicker to increase green-body
  stiffness.
• Special tooling will be required to avoid
  deformations of large green-body parts during
  infiltration.

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Kodak Temperature Measurements
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Kodak Ambient/Cryo Delta (Phase) Map
Mirror Nominal Test Temp. 120K Cryo-Figure
Change 3.9 waves PV 0.45 waves RMS Seidel
Coefficients (1 wave .63 micron) Tilt 0.18
waves PV Focus 0.04 waves PV Astigmatism 1.55
waves PV Coma 0.80 waves PV Spherical -1.99
waves PV
Optical Test Fiducial
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Astigmatic Axis
Optical Test Fiducial
Astigmatic Axis
B
2mm thickness zone
Direction of Decreasing Thickness
Approximate Axis of Warpage from C-SiC Processing
3mm thickness zone
Direction of Decreasing Thickness
A
A
2mm thickness zone
Grain Flow of Residual Carbon
Grain Flow of Residual Carbon
B
Section B-B Astigmatic Shape Change from C/SiC
Processing (cylindrical)
Section A-A Astigmatic Shape Change Going
Cold (cylindrical)
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Astigmatic Axes
Optical Test Fiducial
Optical Test Fiducial
Approximate Axis of Warpage from C/SiC Processing
Grain Flow of Residual Carbon and Astigmatic Axis
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Kodak IABG Cryo-Test Summary

• 0.5m C/SiC demo achieved 120K during cryo-figure
  test
• Primary aberrations are astigmatism and spherical
• Astigmatism attributable to greenbody local layer
  anisotropy becoming dominant in thin
  cross-section of the facesheet
• Single mat layer thickness of the flat greenbody
  is approximately 4mm after densification
• Spherical mirror blank faceplate thickness is
  2.5mm nominal
• Astigmatic behavior is not apparent on 75mm
  diameter, 10mm thick test pieces with balanced
  layers
• Results are encouraging since all aberrations can
  be fixed by processing, i.e. they are correctable
  during cryo-figuring

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Astrium Boostec Silicon Carbide

• Astrium Boostec making SiC mirror for Herschel
  Telescope
• 3.5 m diameter
• Brazing 9 petals
• 1.35 m diameter demonstrator mirror compete
• Areal density 26 kg/m2

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Astrium Boostec Silicon Carbide

• Finely ground silicon powder is mixed with
  organic binders and sintering additives.
• Green body blank formed via isostatically
  pressing at high pressure.
• Green body is similar to chalk and easily
  machined.
• Shrinkage can make it difficult to obtain
  lightweight structures.
• Astrium Boostec appears able to accurately
  predict shrinkage.
• After machining, piece is pressureless sintered
  at about 2000?C
• The resulting substrate can be polished and/or
  ion figured.
• Surface roughness of 3 nm depending on blank
  porosity (less than 3)
• Substrate coated with CVD or PVD SiC for finishes
  of 0.1 to 0.5 nm.
• Sintered blanks can be brazed together to create
  monolithic blanks up to the size of the furnace
  required for brazing (around 3 meters).
•